1. Field of the Invention
The present invention relates to a stencil printing apparatus which wraps a master around a plate cylinder to perform printing, and more specifically to a technology for preventing the occurrence of abnormal images and the occurrence of confidential information leakage associated with the afterimage phenomenon caused by degenerated ink.
2. Description of the Related Art
As a simple printing method, digital heat-sensitive stencil printing has been conventionally known. This allows a thermal head having a plurality of heat generators to come into contact with a master obtained by sticking a thermoplastic resin film and a porous support together, conveys the master using conveying means such as a platen roller while pulsely energizing the heat generating elements, to thereby heat-melt punch a perforated image on the thermoplastic resin film of the master on the basis of image information. Thereafter, the perforated master is wrapped around a porous circular plate cylinder, and a sheet is pressed against the outer circumferential surface of the plate cylinder by means of a press roller or other pressing means to thereby cause ink that is supplied to the inner circumferential surface of the plate cylinder to bleed from the perforation part of a plate cylinder and the perforated part of the master, and to transfer the ink to the sheet to obtain a printed image on the sheet.
In the stencil printing apparatus used in the above-described stencil printing, generally the standby state is held while the master of a preceding plate is wrapped around the plate cylinder, during the period between the time when the printing is ended and the time when the next plate-making operation instruction is issued. The reason is to prevent the occurrence of the following problems when the plate cylinder is left stand for a long time after the master is released: (1) foreign matters might adhere to the exposed outer circumferential surface of the plate cylinder, causing an image failure; (2) the surface layer of the ink might be exposed to the air, changing the ink composition and deteriorating the image quality; and (3) the user of the stencil printing apparatus might mess his/her hands and cloth when detaching the drum from the apparatus main body.
However, letting the apparatus stand for a predetermined amount of time or more while the printed master is wrapped therearound causes a change in ink composition between the perforated section and non-perforated section of the wrapped master. The main example of the ink used in stencil printing is an emulsion ink in which water is dispersed in oil. A change in the composition of this ink hardly occurs between the perforated section and non-perforated section of the wrapped master because, when this ink adheres to the master, the ink remaining on the non-perforated section of the master is protected by the master so that it is not exposed to the air, while the ink remaining on the perforated section of the master is easily exposed to the air and thereby the aqueous phase component of the ink composition evaporates significantly, reducing the ink viscosity and making the ink watery. The difference in the change in ink composition becomes notable as the time for letting the apparatus stand passes, and when the time for letting the apparatus stand exceeds a predetermined time period, the ink of the changed composition creates an afterimage (particularly on the part with a photographic image) when the next plate-making and printing operation are performed, whereby image failures might occur on the first tens of images and some information might leak due to the fact that the contents of the previous original can be inferred from the afterimage.
When, on the other hand, the apparatus is left stand for a relatively short time and the predetermined period is not exceeded, it is understood that letting the apparatus stand while the printed master is wrapped therearound does not affect the next plate-making and printing, but a method for performing detailed control of the presence or absence of a master around the plate cylinder in accordance with the elapsed time has not yet been proposed for the mechanism for generating the afterimage phenomenon.
For these drawbacks described above, Japanese Examined Patent Publication No. S62-30117 (Prior Art 1), for example, discloses a technology in which a security protection mode is provided to prevent the confidential information from leaking a master used in printing, wherein the master of a preceding plate is discharged during the security protection mode to wrap an unprocessed master around a plate cylinder. Also, Japanese Unexamined Patent Publication No. 2001-213039 (Prior Art 2) discloses a technology for preventing the clogging with ink that is caused by leaving a plate cylinder having a master of a preceding plate wrapped therearound for a long time, wherein in the case where the final printing operation is not the confidential plate-making operation, the master on the plate cylinder is discharged after a predetermined time period has elapsed since the final printing operation and an unprocessed master is wrapped around the plate cylinder. Moreover, Japanese Unexamined Patent Publication No. 2005-153477 (Prior Art 3), for example, discloses a technology for securely preventing a third party from reproducing an image using a used master when the security protection mode is set, wherein when the security protection mode is set a master is discharged from a plate cylinder upon completion of a printing operation and a state where the plate cylinder has no master is maintained. Japanese Unexamined Patent Publication No. 2007-55073 (Prior Art 4), for example, discloses a technology for preventing a master from adhering to a plate cylinder due to ink solidification, wherein when a user does not request for plate discharge upon completion of a printing operation a plate-discharging operation is performed automatically.
In the technologies disclosed in Prior Art 1 and Prior Art 2 above, although the effects of these technologies are acknowledged, the problem in these technologies is that a new unprocessed master is required and two masters are actually necessary for printing because an extra plate of master is spent in each printing.
In the technology disclosed in Prior Art 3 above, the plate-discharging operation is performed immediately when the security protection mode for protecting confidential information is set; which prevents an operator from performing additional printing of his/her original and, due to the exposed outer circumferential surface of the plate cylinder even for a short time in which the afterimage phenomenon does not usually occur, foreign matters adhere to the outer circumferential surface of the plate cylinder and cause an image failure, or the surface layer of the ink is exposed to the air, changing the ink composition and deteriorating the image quality. Moreover, a hand or cloth of the operator might be contaminated when detaching the plate cylinder from the apparatus main body.
The technology disclosed in Prior Art 4 above is designed for preventing a master from sticking to the outer circumferential surface of the plate cylinder, so that the plate-discharging operation can be securely performed even when the operator forgets to instruct the apparatus to perform the plate-discharging operation, and so that the operator can secure an extra time for performing printing again by providing a time difference until the plate-discharging operation is performed. However, a master will not be wrapped around the outer circumferential surface of the plate cylinder unless the operator issues a plate-making instruction after performing printing, hence the same problems as the abovementioned “Prior Art 3” occur.
Although a certain effect of each of the conventional technologies is acknowledged in not only the cases that are not aimed at reducing the occurrence of the afterimage phenomena but also the cases aimed at reducing the occurrence of the afterimage phenomena, each conventional technology has the elements that cause the problems, such as the increase in cost of spending masters, cost increase caused by adding new members, decrease in efficiency of an additional printing job, which is caused by frequent plate-discharging operations, and image failure and contamination caused by exposing the plate cylinder.